When a pressure control apparatus used in a super-critical fluid chromatograph (SFC) or super-critical fluid extractor (SFE) is cited as an example of controlled object, this apparatus performs feedback control on a degree of opening of the back pressure-regulating valve of the pressure control apparatus provided downstream of a detector of an analysis flow channel by using a piezoelectric element as a driver element, and controls pressure of a passing fluid (refer to Patent Documents 1, 2).
One feedback control method of a controlled object is a Proportional Integral (PI) control method. The PI control method performs feedback control on the controlled object by using a combination of a proportional component (P) and an integral component (I).
Methods for implementing the PI control method include an analog scheme using an electric circuit and a digital scheme that performs digital processing by software and the like (refer to Patent Documents 3, 4).
Among these schemes, the analog scheme may need a complicated, large-scale control circuit depending on the controlled object as will be described later, and control may become unstable. On the other hand, the digital scheme may not satisfy responsivity depending on the controlled object because A/D conversion, D/A conversion, and digital processing take time.